Observing variation of acoustical characteristics of several common firearms in a quasi anechoic environment at a high sampling rate

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Abstract

Audio recordings from a shooting incident may provide crucial information for a criminal investigation. A typical gunshot signal includes two high amplitude and short duration impulsive signature sounds, the muzzle blast, observed in all the gunshot waveforms, and the bullet s shock wave, which can only be detected if the bullet travels at supersonic speed. Acoustic gunshot analysis generally focuses on the study of muzzle blast signals, which last only a few milliseconds. Ideally, gunshot signals needed to be record at a very high sampling rate to reveal the muzzle blast details. Real life gunshot recordings are record with equipment not designed for these high-amplitude sounds. Moreover, the recordings contain the direct sound of the gun along with multiple overlapping signals due to sound reflections from the ground, nearby surfaces, and other obstacles. The resulting reverberant recording may be difficult to interpret. To study the details of these signals in a scientific manner, we have developed a quasi-anechoic procedure to capture gunshot signals at a very high sampling rate (500 kHz samples per second) using 12 microphones covering 180° in azimuth. The recordings are made in an open air environment with a raised shooting platform and microphone position, resulting in sufficient delay between the arrival of the direct sound at the microphones and the arrival of the first reflection (from the ground). The firearms used in this experiment include a Remington 870 shotgun, 308 Winchester rifle, AR15 rifle, and a 22LR rifle. Handguns tested include a Colt 1911A1, Glock 19 with 9mm ammunition, Glock 23, Sig 239, and a Ruger SP101 with both 357 Magnum and 38 Special ammunition. A number of successive shots were record for each of the firearm type. Based on analysis of the recorded data, we find that acoustic gunshot signals vary from one firearm to another in terms of peak sound pressure and Muzzle blast duration. For a given firearm, we observe significant differences in sound level and also Muzzle blast duration as a function of azimuth and find that there is measurable variation in signal details among successive shots from the same firearm.